Global navigation satellite systems (GNSS; satellite navigation system for short) are used for position determination and navigation on the ground and in the air. GNSS systems, such as the European satellite navigation system currently under construction (also referred to hereinafter as the Galileo system or Galileo for short), comprise a satellite system (space segment) comprising a number of satellites, a receiving means system (ground segment) on the surface of the earth, connected to a central computation station and comprising a plurality of ground stations and Galileo sensor stations (GSS), and user systems, which evaluate and make use of the satellite signals transmitted by the satellites by radio communication, in particular for navigation. From the space segment, each satellite emits a signal characterising the satellite, the Signal in Space (SIS). The SIS comprises in particular data on the orbit of the satellite and a timestamp for the time of transmission, which are used to detect the position of a user or user system.
Precise detection of the position of a user requires integrity in a GNSS. In particular, Integrity firstly means that the GNSS is capable of warning a user within a particular period of time if parts of the GNSS are not to be used for navigation, for example if system components fail, and secondly that the user can have confidence in the navigation data which he receives by means of satellite navigation signals from the satellites of the GNSS, and in particular can rely on the accuracy of the received navigation data.
The Galileo integrity concept provides that the following data are transmitted to user systems by means of the navigation signals:                data concerning the accuracy of the transmitted navigation signal for each satellite, i.e. a signal in space accuracy (SISA) for satellites as a measure of the quality of an SIS of a satellite;        status reports on the accuracy of the monitoring of the satellite by the ground segment, i.e. a signal in space monitoring accuracy (SISMA) for each satellite; and        an integrity signal in the form of a simple error display for an incorrect SIS of a satellite “Not OK” (known as the integrity flag IF) and the threshold for reporting that the error in an SIS of a satellite is no longer acceptable, also known as the IF threshold.        
These data make it possible for a user system itself to quantify and evaluate the integrity and the integrity risk.
For Galileo, the SIS's of the satellites are monitored within the ground segment by evaluating the measurements of the individual Galileo sensor stations (GSS). The GSS measurements are processed in a central integrity processing site in the ground segment to determine the integrity data, listed above, which are to be broadcast to the user systems.
Using the known positions of the GSS's in the integrity processing site, the current position, the instantaneous deviation of the broadcast timescale from the system timescale, and the signal properties of a satellite, and thus the maximum error of the satellite or the signal emitted by said satellite in space, known as the signal in space error (SISE), are estimated.
A prediction of the distribution of the SISE can be represented by a normal distribution with the smallest possible standard deviation. This prediction is represented by the aforementioned signal in space accuracy (SISA), which is broadcast from the ground segment to the user systems via the satellites of the space segment. Using the SISA it is possible to describe the difference between the current 4-dimensional position (orbit and time) of a satellite and the predicted 4-dimensional position found in a navigation message.
However, estimating the SISE is an error-prone process. For this reason, it is generally assumed that the distribution of the current SISE about the value of the estimated SISE can be described by a normal distribution with the standard deviation represented by the aforementioned signal in space monitoring accuracy (SISMA). SISMA is thus a measure of the accuracy of the estimate of the SISE for a satellite in the ground segment and is also transmitted to the user system from the ground segment via selected satellites of the space segment. In the Galileo system, the SISMA values for the satellites are transmitted approximately every 30 seconds. To minimise the integrity risk, the respective largest SISMA value for each satellite out of the SISMA values determined in a measurement period is transmitted. A large SISMA value in this case represents a low accuracy of the satellite monitoring by the ground segment and thus reflects an increased integrity risk for a user.
A detailed description of the Galileo integrity concept can be found in the publication “The Galileo Integrity Concept”, V. Oehler, F. Luongo, J.-P. Boyero, R. Stalford, H. L. Trautenberg, J. Hahn, F. Amarillo, M. Crisci, B. Schlarmann, J. F. Flamand, ION GNSS 17th International Technical Meeting of the Satellite Division, 21-24 Sep. 2004, Long Beach, Calif.